Viscosity measurement

ABSTRACT

The present invention relates to a method and apparatus of determining the rheological properties of a polymer flowing in a conduit. The invention provides a method of characterising a polymer under test, comprising: Detecting acoustic emissions from said polymer flowing in a conduit to provide acoustic emission data, comparing the acoustic emissions data obtained against acoustic emission data from a polymer, or a series of polymers, of known characteristics, and thereby characterising the polymer.

This is the US national phase of international applicationPCT/GB01/05771 filed 21 Dec. 2001, which designated the US.

The present invention relates to a method of determining the rheologicalproperties of a polymer flowing in a conduit. The method is particularlysuitable for determining the viscosity of the polymer, and otherproperties such as the molecular structure or chemical composition ofthe polymer can also be determined. Also provided is an apparatus forcarrying out the method.

Polymers are generally manufactured using chemical synthesis reactionsbetween one or more basic molecules, known as monomers, which reacttogether under favourable conditions to form a polymer, which consistsof long chains of the monomers joined together.

In general, in polymer manufacturing processes, the composition of thepolymer chain (i.e. the molecular structure of the polymer) is carefullycontrolled by adding the monomer(s) to the reaction mixture at acarefully controlled rate. Where there are two or more monomers, theseare added to the reaction mixture in strictly controlled proportions toone another e.g. in a constant ratio. It is also necessary to maintainthe reaction conditions at the correct levels in order to control therate at which each monomer reacts with the other monomer(s) and hencecontrol the resulting molecular structure. Reaction conditions includetemperature, pressure, rate of mixing, rate of shear etc. Of course,even when a single monomer is used, such as in the manufacture ofpolyvinyl chloride (PVC) or polyethylene, the molecular structure isalso affected by the reaction conditions because the length of eachpolymer chain can vary and the chains can be branched or unbranched tovarying degrees. This degree of “branching” of the polymer chain affectsthe physical properties (e.g. density and strength) of the polymerproduct.

Clearly then, the molecular structure of a polymer product must becarefully controlled during polymerisation reactions. However,measurement of the polymer properties during reaction is extremelydifficult. Properties such as the viscosity or melt flow index (MFI) ofthe polymer when melted are very good indicators of the molecularstructure, and hence the physical and chemical properties of thepolymer. However it is necessary either to take samples of polymer fromthe reactor in order to carry out conventional measurements of viscosityand MFI in the laboratory, or in some cases an “on-line” rheometer maybe fitted in the outlet pipe from a reactor.

Sampling techniques are time-consuming and introduce delays in obtainingthe information—therefore this is not an effective way of continuouslycontrolling the reaction since by the time the results are analysed andappropriate action taken, the reaction conditions will be different andsome of the polymer product may already be adversely affected.

On-line rheometers generally work on the principle that a small amountof molten polymer is syphoned off into a smaller “by-pass” duct, and therheological properties of the polymer, such as the MFI or viscosity canbe measured. The rate of flow of the polymer in the by-pass line at agiven pressure (or load) is dependent on the viscosity or MFI of thepolymer, at a known shear rate. Hence on-line viscosity or MFI can bemeasured. Unfortunately, though, this form of measurement istheoretically complicated and involves the use of sophisticated andexpensive equipment for example transducers may be needed to measurepressure and flowmeters and sometimes also gear pumps are required.

Another approach to this problem is found in GB 2 038 051, published in1980, which discloses the idea of an “acoustic probe” which can beimmersed in polymerising mixture in the reactor and used to monitor therheological properties of the polymer. The probe was intended to pick upsound-wave signals from the polymer flowing inside the reactor, andamongst other things, it was intended to help to monitor the viscosityof the polymer by correlating viscosity with the logarithmic decrementof sound-wave oscillations.

However, in order to pick up useful measurements, the probe needed to bepositioned in a region of polymer flow, such as near to the stirringdevice in the reactor. This creates practical difficulties in that theprobe is liable to be damaged and is difficult to maintain in positionin the reactor. Any device which has to be immersed in the polymer meltitself is inherently difficult to operate and is generally best avoidedwherever possible. Furthermore, measurement of polymer properties in thereactor has problems because the properties in the reactor are notnecessarily the same as the properties of the final polymer produced.

In Esbensen et al (1998); “Acoustic chemometrics; from Noise toinformation”, Chemometrics and and intelligent laboratory systems 44(1998) 61-76, an acoustic device is described for use with particulatematerials.

Viewed from one aspect, the invention provides a method ofcharacterising a polymer under test, comprising:

detecting acoustic emissions from said polymer flowing in a conduit toprovide acoustic emission data,

comparing the acoustic emissions data obtained against acoustic emissiondata from a polymer, or a series of polymers, of known characteristics,and

thereby characterising the polymer.

Preferably, said method enables a rheological property of a polymerunder test to be determined, by comparing the acoustic emissions dataagainst such data from a polymer, or a series of polymers, of knownrheological properties, and thereby determining the rheological propertyof said polymer under test.

It will be appreciated that in order for the polymer to flow and ameaningful evaluation of its properties to be deduced, it will generallybe necessary to melt the polymer so that it is no longer in solid form.Hence reference to a “polymer flowing” as used herein should beunderstood in general as reference to a molten polymer which is able toflow.

Thus, the invention is based on the discovery that it is possible todetermine the characteristics, preferably the rheological properties, ofa polymer flowing in a conduit, without the need for expensive andcomplex equipment and without the need to immerse a probe or sensor inthe flowing fluid. It has furthermore surprisingly been found that theacoustic emissions from a particular polymer are sufficientlycharacteristic for each different type of polymer to be identified.Also, for any given polymer for which molecular structure may differfrom batch to batch or over time during continuous processing, thisvariation can be monitored. In fact, the composition of the polymer canbe determined from the acoustic emission data of that polymer.

The composition of the polymer can of course be inferred or determinedfrom any values of the rheological properties obtained, e.g. from theviscosity of the polymer, but it will be appreciated that directcomparison of emission data alone from polymers of known identity canalso be made. Thus, in order to identify a particular polymer accordingto its composition, a value for the viscosity or other rheologicalproperty of that polymer need not actually be determined from theacoustic emission data in order to identify the polymer.

Thus viewed from another aspect, the invention provides a method ofidentifying a polymer under test, comprising:

detecting acoustic emissions from said polymer flowing in a conduit,

comparing the acoustic emissions data obtained against acoustic emissiondata from a polymer of known identity, and

thereby determining the identity of the polymer under test.

In this case, the identity of the polymer may be in the form of anaccurate determination of the molecular structure of the polymer, or itmay be simply be an indication of the type of polymer being produced(e.g. determining whether it is polyethylene, polypropylene or aparticular co-polymer, or even the particular composition).

Rheological properties as referred to herein include viscosity(intrinsic, extrinsic, kinematic or dynamic viscosity), shear-strain orshear-stress, melt flow index (MFI) or any other rheological parameterwhich is characteristic of a given polymer. [The term “rheologicalproperty” as used herein however does not include parameters such asflow rate or flow velocity, temperature, pressure, load or pressure dropwhich may or may not be determined incidentally when the method of theinvention is carried out. These and many other properties of a fluidflowing in a conduit are not “rheological properties” within the meaningof the invention since they are not characteristic of any given fluid orpolymer].

It will be appreciated by those skilled in the art that rheologicalproperties are generally determined for a given fluid at apre-determined or preferably constant value of the non-rheologicalproperties. Thus for example the viscosity of a fluid may vary withtemperature, flow rate, pressure etc., hence a value of viscosity shouldideally be compared against another at a given temperature and undergiven flow conditions etc. Since it is the molecular weight andmolecular weight distribution (MWD) which is of prime interest incontrolling the properties and hence quality of the polymer product, itis a change in any of these properties which is of interest rather thanmeasurement of an absolute value, in most cases.

In fact, the viscosity of a polymer also varies with other rheologicalproperties, e.g. shear stress. If a graph of shear stress againstviscosity is plotted for a given polymer, the shape of the curve isindicative of the molecular weight distribution of the polymer. However,by comparing the acoustic emission data obtained in accordance with theinvention against emission data from known polymers under the same flowconditions e.g. at a given temperature, flow rate etc., complexcalculations of the polymer properties can be avoided and the identityand/or rheological properties of a polymer can be determined directly.

It is therefore preferred that the method of the invention be performedby detection of acoustic emissions from the polymer at a pre-determinedflow rate, pre-determined pressure and/or a pre-determined temperature.In particular, it is advantageous to control the flow rate of thepolymer in order that the shear rate of the polymer is known. Forexample, the flow rate of the polymer may be controlled over apre-determined range corresponding to a desirable shear rate range forthe polymer under test. In this way, it is possible to optimise the flowrate to provide a shear rate in which the best possible distinction inmeasured characteristics (e.g. viscosity) is obtained for any givenpolymer. The skilled person will readily understand how to determine theoptimal flow rate range by carrying out simple tests at differentmeasured flow rates. The optimal flow rate range for any given polymerwill depend on the characteristic of the polymer which is to bedetermined.

Apparatus to measure the temperature of the polymer in the conduit iswell-known in the art and may for example be a thermocouple devicecontained in or placed on the conduit. Alternatively, the temperature ofthe conduit in which the polymer flows can be measured either at or nearto the point at which the acoustic sensor is located, or at anotherconvenient point e.g. at the nozzle outlet of an extruder. All that isrequired is that the temperature should be pre-determined at a givenpoint which is indicative of (i.e. related to or dependent on) thetemperature of the polymer at the point where the acoustic emissions arebeing detected.

In many cases, the temperature at which a polymer melts will besignificantly above ambient temperature. Typically, temperatures of apolymer melt may exceed 100° C. and may be as high as 125 to 250° C. orhigher. The sensor means used to direct the acoustic waves emitted fromthe polymer must therefore in many applications be able to withstandthese high temperature.

A typical acoustic sensor means for use in accordance with the inventionwould be an accelerometer. Accelerometers are known acoustic sensordevices and are widely available, for example of the type manufacturedby Brüel and Kjær in Denmark. Where high temperatures need to bewithstood by the sensor means, this should be borne in mind whenselecting a suitable device. Accelerometers for example can bemanufactured to withstand temperatures up to and above 250° C. and thetechnology to do this is well known to manufacturers of accelerometers.

The conduit in which the polymer flows may take any form. Preferablyhowever the conduit is a pipe e.g. a cylindrical pipe which may be madeof any suitable material. Steel is typically used in polymer productionprocesses but other corrosion-resistant materials may be used. Thematerial of the conduit should however be suitable to allow acousticwaves to be well conducted in order to be detected outside the conduit.Hence acoustically conductive materials, especially metals such as steelare preferred. The acoustic sensor means must be placed in acousticalcontact with the conduit.

In order to enhance the acoustic emissions from the polymer as it flows,it is preferable to cause a disturbance in the flow of the polymer inthe conduit. For example, the pipe may be modified in some way to alterthe flow characteristics, especially to cause a sudden change in theflow. Thus, a structural detail may be provided in the conduit in orderthat the conditions of flow change, at or near the portion of theconduit in which acoustic emissions are detected. It has been found thatthe presence of a constriction in a pipe is particularly suitable. Thediameter of the constriction is not crucial but it must be sufficientlysmall relative to the diameter of the conduit to allow the necessarydegree of turbulence to occur. An orifice plate of the type routinelyused for flow measurement is an ideal way of providing a constriction ina pipe. Other forms of structural detail which may be used to createturbulence include, but are not limited to, a bend (e.g. 45° or 90°) inthe conduit, the presence of a valve or other choke mechanism. A suddenincrease in pipe diameter may also be suitable.

Where the polymer exits the reactor in molten form (e.g. low densitypolyethylene) the conduit may be an exit pipe directly from the reactor,or it may be a by-pass pipe from one of the main polymer pipelines.Where the polymer is initially in solid form (e.g. granules or powder) amelting step is needed. The acoustic rheometer in accordance with thepresent invention may be used in a similar manner to existing or knownrheometers i.e. it is suitable for use in any form of conduit andtherefore it may simply replace an existing rheometer. For example,existing and known rheometers such as online rheometers are oftensituated in a by-pass line from an extruder or they may be placed on anextruder directly. For example, the conduit in accordance with theinvention may be associated with a single or plural screw extruder.

As mentioned above, flow conditions are also preferably kept at apre-determined level in order to allow effective comparison of acousticemission data with data from known polymers. Hence, preferably the flowrate of the polymer in the conduit is measured and/or monitored at ornear the point at which the acoustic sensor is positioned. Flow ratescan conveniently be measured by any method known in the art i.e. by anyflowmeter, but it may in some instances be convenient also to measureflow rates by acoustic means, e.g. by detecting the Doppler shift etc.However, in order to measure the flow rate in accordance with suchapparatus, it will be noted that a sound-wave (ultrasound >25 kHz)source other than the polymer flow itself must be present, as thistechnique depends on detection of ultrasound waves which are reflectedoff the flowing fluid.

This differs from the detection method of the present invention whichrelies on passively emitted acoustic waves from the polymer itself.However, there is no reason why any necessary flow rate measurementscannot be taken using a separate ultrasound sensor means insender-receiver mode, and utilising this ultrasound sensor means to pickup the reflected ultrasound for flow rate measurement.

Where the polymer is passing through a pipe or extruder, the pressure inthe extruder or pipe is also preferably measured and/or maintained at apre-determined level.

As explained above, the invention relies on the principle that movementof the polymer, for example through a constriction in the conduit,causes the polymer-conduit assembly to produce vibrational acousticemissions, which can then be detected. One preferred way in which thedetection takes place is to generate an acoustic spectrum whichtypically may take the form of a graphical representation of the emittedacoustic waves. An example of an acoustic spectrum is shown in FIG. 5.However, in its simplest form, an acoustic spectrum generated inaccordance with the invention could take the form of a plot of amplitudeon the Y axis against frequency on the X axis called a “power spectrum”.

The acoustic spectrum for any given polymer acts as a multivariant“fingerprint” for that polymer, since it is different from the spectraof other polymers (and other fluids generally) flowing at the same pointin the conduit under the same flow conditions. Hence, in accordance withthe invention, a polymer can be identified by comparing its acousticspectrum against acoustic spectra of known polymers until a match isfound. Where the rheological properties of that polymer are also known,the rheological properties of the polymer under test can also bedetermined from a comparison of the acoustic spectra.

If the acoustic spectra are recorded e.g. in electronic form or in anyother form of searchable database, rapid comparison of data can becarried out e.g. by computer analysis, and swift matches for theidentity of a polymer and/or the rheological properties of a polymer canbe found. The speed of response which can be achieved using computerprocessing techniques means that data obtained from the detection ofacoustic emissions can be analysed in a database, and values forrheological properties or the identity of a polymer can be determined ina matter of seconds, or even milliseconds. Hence the method of theinvention is particularly advantageous for on-line monitoring ofproperties of polymers and this can be used to facilitate processcontrol.

Thus in a preferred aspect, the invention provides a method for thedetermination or on-line measurement of the rheological properties of apolymer, comprising:

detection of acoustic emissions from said polymer flowing in a conduit,and

comparison of the acoustic spectrum generated against the acousticspectra of polymers of known rheological properties, whereby todetermine the rheological properties of the polymer under test.

The range of acoustic emissions detected may be anywhere in the acousticfrequency range of 0 to about 25 kHz.

As explained above, the acoustic emissions detected can provide a set ofdata which can provide a “fingerprint” of the polymer concerned.

In a simple case, acoustic emission spectra can provide a set of numberswhich is characteristic of the particular polymer produced. This set maybe compared with a corresponding set which is known to relate toacceptable products (e.g. from previously produced product). Bydetermining whether the numbers are sufficiently similar (e.g. withinpreviously specified tolerances) it may be determined whether the fluidis itself acceptable. It will be appreciated that these numbers relateindirectly, but unambiguously to molecular weight and molecular weightdistribution, although absolute values need never be found for theseparameters. Nevertheless, it may in practice also be useful to do so.

The previously acquired sets of acoustic emission data may have beenobtained by making similar measurements of known polymers having desiredcharacteristics. For example, sets of data may be obtained for eachpolymer which it is desired to produce corresponding to the idealconditions for producing that polymer.

Close similarity between the measured data and one of these previouslyacquired sets of target data may then be used to identify the polymerconcerned and/or to determine whether a desired polymer is beingproduced with the correct characteristics.

It will be appreciated that this comparison could be performed innumerous ways and in the simplest case useful information could beobtained even from visual comparisons of plots of the various data sets.However, these comparisons are preferably automated. In practice thismeans that the comparisons are carried out by a computer.

Numerous known computational techniques may be used to perform theanalysis, but it is has been found that multivariate calibration isparticularly effective and accurate (see Martens and Naess 1989“Multivariate Calibration” published by John Wiley, Esbensen (1998)).Thus, a latent variable corresponding to an optimal linear combinationof the measured frequency data may be introduced. The data are thenredefined in relation to this latent variable.

In a particularly preferred form of the invention, Principal ComponentAnalysis (PCA) of the acoustic emissions data is used for classificationof new samples in relation to old samples of known properties. The rawdata may be subjected to preprocessing such as e.g. transformation,centering, smoothing or scaling. Subsequently, from a set of samples(“calibration set”) of known properties a data subspace is empiricallyidentified into which the test sample data points may be projected. Thissubspace is described by a set of “latent variables”, spanningindividual axes in the subspace and is denoted the “model” of the givenclass of samples. The number of latent variables are then empiricallyfound as those needed to give representative information related to flowproperties of the fluid in question based on casual knowledge by theoperator. It will be noted that it is not necessary to run anytransformation to align with rheological parameters.

If a visual evaluation is desired, a plot of the data may be producedwhere the axes are given by the latent variables, and where new samplesare compared to the set of known samples, and to limiting values basedon the same samples. For a mathematical evaluation (classification)upper and lower limiting values may be defined for the value of thelatent variables, and for residuals of the raw data after projectinginto the subspace an upper limiting value is defined. Then new samplesmay then be classified according to these limiting values. This approachhas been termed the SIMCA approach, as referred to in Esbensen 1998 andnumerous other references herein.

Typically, when using PCA, the latent variables are defined by theeigenvectors of the (n×k) matrix e.g. where n is the number of samplesin the calibration set and k is the number of values measured for agiven variable. Each sample in the calibration set, and future testprocess samples, may then be described by their score values along theindividual latent variables thus defined.

By calculating the correlation of the latent variable with polymerproperty parameters like MWD, MFR (melt flow rate), etc. one will obtainknowledge of along which direction these parameters have their largestvariability in the latent variable data space. This information may becompared to the position of the individual samples in the same dataspace, to evaluate their score in relation to the different parameters.

By calculating the correlation of the latent variable with processingparameters like reactor temperature, reactor feed compositions etc., onewill obtain knowledge of along which direction these parameters havetheir largest variability in the latent variable data space. Thisinformation may be compared to the position of the individual samples inthe same data space, to evaluate their score in relation to thedifferent parameters, and it may be used to estimate how processparameters should be changed to change the positioning of the product inthe latent variable space to have the selected flow propertiesrepresented by the acoustic emission data values.

It is particularly preferred for the method to be implemented using acomputer arranged to display a score plot representing the data at leastsubstantially in real time. In this way, as new data is acquired and newplots are added to the score plot, changes in the fluid (polymer)characteristics may be followed. It is helpful for an indication to beprovided on the display of where the boundaries between acceptable andunacceptable points lie, for example based on statistical quantities.The indications may be a boundary line in the form of an ellipse. Pointsfalling outside the boundary correspond to unacceptable product.

As discussed above, the score may be evaluated in relation to differentparameters and so it is possible to correlate the position of a pointoutside the boundary with the corresponding deficiency in itsproperties. This information may then be used to enable appropriatecorrective action to be taken by a plant technician. For example, thepreviously acquired data sets could include data corresponding to knownincorrect settings for the desired product from which previouslydetermined corrective action may be taken. Such previous data sets couldhave been deliberately produced or they could be learned automaticallyfrom analysis of previous operations of the plant. Alternatively theplant may be adjusted in an iterative manner based upon the nature ofthe deviation of the measured data sets from the desired data set.

In particularly preferred forms of the invention, means is provided toautomatically adjust the operating conditions of the plant in order toameliorate the deficiency. Of course, there need not be a display forthis to be effective—the “ellipse” may simply be a defined volume ofdata space.

Another advantage of this form of the invention is that even if aproduct is determined to be acceptable, it is possible to monitorvariations in where points are plotted (or located in data space) inorder to determine trends which may be used to anticipate futuredeficiencies and to take corrective action before they occur. Preferablythis is also implemented automatically.

In this context PCA represents one way of identifying the latentvariables. However, it will be appreciated that any other mathematicalmethod involving linear or non-linear transformation of the relevantprocess data into a set of latent variables may be used. Examples ofother methods are Partial Least Squares Regression (PLSR), NeuralNetworks (NN) and curve fitting of the pressure data or preprocessedpressure data to a curve of selected exponential degree.

A particularly preferred aspect of the invention is to use the acousticemission data for quantification of selected polymer properties, e.g.MFR or MWD. Again the raw data may be subject to preprocessing such ase.g. transformation, centering, smoothing or scaling. From a set ofsamples (“calibration set”) of known properties it is then possibleempirically to identify a mathematical relation (the “model”) toquantify the selected properties based on the preprocessed pressure.This model may be any linear or non-linear relation defined by methodslike Principal Component Regression (PCR), Partial Least SquaresRegression (PLSR), Neural Networks (NN), etc.

When using PCR and PLS, latent variables may be identified in a modifiedform closely related with PCA (above), and then a linear regressionmodel is developed between the polymer property and this type of latentvariable. In the same way as when doing classification above, the scorevalues in the latent variable space may then be used for visual andmathematical evaluation. Correlation between the latent variables andprocess parameters may be used to identify how the process parametersshould be changed to adjust the selected property of the polymer beingproduced.

It will be appreciated from the foregoing that the present invention isuseful in the field of polymer production and so acoustic emissiondetection means is preferably situated on-line and may be associatedwith an extruder used in such a context. Polymer may be fed from theextruder directly into a suitably modified conduit for acousticemissions to be detected, e.g. by means of a bypass. Because of thespeed of operation and the improved accuracy of the method of theinvention, if the properties of the polymer are as desired, this will beknown much more speedily than in the prior art system. Furthermore, itis also possible to determine more quickly if the measuredcharacteristics are not as required and then to adjust the operatingconditions of the reactor accordingly in order to obtain the desiredcharacteristics. Consequently, wasted production may be greatly reduced.

It is possible to apply the method of the present invention either onlywhen the reactor is first set up for a given production run, or atoccasional intervals as required by quality control. However, since themethod may operate automatically it is particularly preferred thatregular and comparatively frequent measurements be made, say aroundevery 10 minutes.

Polymer producing plants are normally operated continuously and if it isdesired to change from production of one polymer to another this is donewithout closing down the plant. Instead, the reactor operatingconditions are adjusted in order to change the polymer thereby producedand fed to the extruder. Thus, preferably the method of the invention isused to obtain data which is used to monitor the transition betweenproducts. Since in the preferred forms of the invention the dataacquisition and subsequent comparison steps are carried out by computer,this may be done rapidly. Consequently, the transition may be effectedmore smoothly and quickly than in the prior art and moreover theoperator can determine more quickly when the desired product starts tobe produced. It will be appreciated that this significantly reduces theamount of wastage associated with operation of the reactor therefore asignificant advantage in terms of saving time and materials and therebycosts.

The invention also provides an apparatus, also referred to herein as anacoustic rheometer, for carrying out the method of the invention, andthe use of the acoustic rheometer to control a polymerisation reaction.Thus viewed from a further aspect the invention provides an apparatusfor characterising a polymer, comprising:

a) an acoustic sensor capable of detecting acoustic emissions from thepolymer and thereby generating a signal;

b) means for comparing the signal against acoustic emissions data frompolymers of known characteristics. This data may for example be storedin a computer memory either provided within the apparatus or remotely.

The invention also provides the use of an acoustic rheometer comprising

a) an acoustic sensor capable of detecting acoustic emissions from apolymer;

for controlling a polymerisation reaction producing said polymer.Preferably, in this aspect, the acoustic rheometer further comprisesmeans for comparing the signal against acoustic emissions data frompolymers of known characteristics, as defined above.

Preferably, the apparatus is adapted for determining the rheologicalproperties of a polymer, comprising:

a) an acoustic sensor capable of detecting acoustic emissions from thepolymer and thereby generating a signal;

b) means for comparing the signal against acoustic emissions data frompolymers of known rheological properties whereby to determine a valuefor the desired rheological property of the polymer under test.

The apparatus may further comprise means for identifying the polymer.Preferably, the apparatus comprises an acoustic sensor which is capableof detecting vibrational acoustic emissions in the interval 0-25 kHz.

The acoustic sensor may be as described above. The means for comparingthe signal (referred to hereinafter as “comparison means b)”) may ifnecessary or desired comprise means for amplifying or processing thesignal from the acoustic sensor. For example the comparison means b) maybe a computer which in turn may be connected e.g. to a signal amplifieror preprocessor. The computer will preferably be loaded with suitablesoftware. Conveniently, the comparison means b) may be provided by apackage such as the Multi-Channel Spectrum Analyser available fromApplied Chemometrics Research Group (ACRG), Tel-Tek, Porsgrunn, Norway.

The apparatus of the invention is set up such that the acoustic sensormeans is positioned in acoustic contact with (preferably touching) theconduit through which a polymer can flow. The conduit is preferably astraight segment of a pipe and preferably this has a structural detailas hereinbefore described. The acoustic sensor means is thereforepositioned whereby to detect acoustic emissions from the flowing polymeras it passes through the structural detail in the pipe.

It has been found in particular that the acoustic sensor means can beplaced in a variety of positions in relation to the conduit in order tosuccessfully determined rheological properties of a polymer. Forexample, it could be placed before or after the structural detail e.g.within about 5-20 cm or 5-10 cm of the structural detail (relative tothe direction of flow) but preferably it should be placed before thestructural detail. Alternatively it could be positioned at the positionof the structural detail itself, which is particularly preferred.

The invention also extends to a polymer production plant incorporatingthe method or apparatus of the invention as set forth above and also topolymer products thereby produced.

Certain embodiments of the invention will now be described, by way ofexample only and with reference to the accompanying drawings in which:

FIG. 1 is an acoustic rheometer according to the invention;

FIG. 2 is a schematic flow diagram showing the data path for analysis ofthe acoustic emissions data;

FIG. 3 is a diagram of one possible configuration of an acousticrheometer according to the invention;

FIG. 4 is another diagram showing a different possible configuration ofan acoustic rheometer according to the invention;

FIG. 5 shows an acoustic spectrum in the frequency range 0-25 kHz, foreach of the four different polymers as described in Example 1.

FIG. 6 shows a score plot acoustic spectrum for PCA (principal componentanalysis) of these four polymers. The percentage score of Component 2(22.3%) is plotted against the percentage score of Component 1 (35.7%).

FIG. 7 shows the PLS model (partial least squares), with the viscosityof the modelled values plotted against the measured viscosity for eachof the polymers.

FIG. 8 shows the variation in viscosity of the four different polymersas measured by a rheometric dynamic analyser in a frequency sweep mode(190° C. melt temperature). On the Y axis the crossplot viscosity isgiven at 300 rad/sec and on the X axis it is given at 0.05 rad/sec.

FIG. 9 shows a score plot (t1t2) of five replicates of each of thepolymers designated A, B, C and D.

FIG. 1 shows an ultrasound rheometer for operation in accordance withthe invention. Typically, the polymer melt leaving the polymerisationreactor will be processed through an apparatus 1 which consists of aconduit 2 with a constriction 3 allowing the polymer to pass through.The acoustic sensor means 4 e.g. an accelerometer may be placed in anyone or more of positions A (before the constriction), B (at theconstriction) or C (after the constriction). The accelerometer 4 detectsacoustic emissions from the polymer flowing through the apparatus 1 andgenerates an acoustic spectrum which is characteristic of the polymer.The signal is amplified by an amplifier/preprocessor 5 and data analysisis carried out by a computer 6 or other suitable means. Data analysiscan for example be carried out by mulitvariate analysis techniques suchas principle component analysis (PCA) or partial least squares (PLS).Information on the viscosity, molecular structure, MFI and other polymerproperties can then be calculated by comparison with information fromknown polymers.

FIG. 2 is a schematic flow diagram showing the data path from theacoustic emissions (vibrations) generated by the polymer and how thatdata is analysed numerically. Box 7 represents polymer flow through theconstriction in the conduit, from which the acoustic signal is detectedby the sensor accelerometer 8. Box 9 represents signal processing byadaptation of the signal through a lowpass filter and analog-digitalconversion to allow analysis of the signal. Multivariate analysis of thesignal data is then carried out, as represented in box 10.

FIG. 3 shows one possible configuration for the acoustic rheometermounted on a by-pass from an extruder. The extruder barrel 11 is shownwith the by-pass line 12 leading from it and round the by-pass “loop”back into the extruder 11. Polymer is pumped round the by-pass pipe 12by means of a gear pump 13, piston, or any other suitable device forgenerating flow, and through a constriction 14 in the by-pass pipe. Theacoustic sensor means 15 is placed outside the pipe in acousticalcontact therewith, and leads 16 transmit the signal to an amplifier 17and then to a personal computer 18 which is capable of analysing thedata by means of multivariate analysis (MVA).

FIG. 4 shows another possible configuration for an acoustic rheometer.The polymer process flow from the polymerisation reactor is in powderform and is transported through pipe 19 from the reactor. A portion ofthe polymer is drawn off from the main flow pipe at a samplingpoint/system 20 and passed through a single screw extruder 21 or anyother suitable device where it is heated and melted to allow it to flow.An acoustic sensor means 22 is placed in acoustical contact with thesingle screw extruder pipe at a point before where the polymer flowsthrough a constriction 23. The signal detected by the acoustic sensormeans 22 is transmitted to a data analysis unit such as a computer (notshown).

EXAMPLE 1

Comparison of 4 Different HDPE Resins using Acoustic Rheometer

The aim with this study was to compare the acoustic spectrum recorded asdescribed in the patent with viscosity data obtained using aconventional rheometer (plate—plate dynamic rheometer; Rheometricsdynamic spectrometer, RDA-II) For this purpose 4 commercial HDPE (highdensity polyethylene) polymers manufactured by Borealis were chosen:

-   -   HE8168, HE8343, LE7520, LE0400

Viscosity data obtained by the dynamic rheometer (at 190° C.) is shownin table 1 below:

TABLE 1 Viscosity vs. shear rate at 190° C. viscosity (Pa · s) atdifferent shear rates polymer shear rate LE7520 LE400 HE8343 HE8168 25511 1986 3793 860 38 419 1516 2973 857 50 368 1269 2534 855 74 306  9852017 840

The experimental setup for the rheometer is shown in FIGS. 1 and 2;

The rheometer is basically a heated pipe in which a die is inserted inorder to create a constriction in the pipe. At the flow inlet of the dieis placed an accelerometer in order to record sound generated by theflowing polymer. Polymer is being fed by a 30 mm extruder (manufacturedby company Collin GmbH)

The procedure of collecting and numerically treating the data is shownschematically in FIG. 2. (for further explanation refer to Esbensen etal (1998); “Acoustic chemometrics; from Noise to information”,Chemometrics and and intelligent laboratory systems 44(1998) 61-76.

Each polymer was extruded at 4 rpm's (30, 45, 60, 90). With the diechosen for the experiments (7 mm diameter) these rates equal shear ratesas shown in table 1. In table 1 (above) viscosities for the 4 polymersat the given shear rates are given based on laboratory measurements.During the experiment with the acoustic rheometer, the following datawere recorded:

-   -   polymer temperature (end of extruder)    -   polymer pressure    -   Polymer temperature at the measurement point    -   acoustic spectrum (FIG. 5)        MVA Analysis

A plot of the different acoustic spectra is shown in FIG. 5 A PCA(principal component analysis) analysis is shown in FIG. 6: the scoresof the first two latent variables show that the spectra are able todistinguish between the different polymers in a systematic manner.

By combining table 1 and the recorded spectra we can use PLS regressiontechnique to study how the acoustic spectra explain the variation onviscosities at the actual shear rates.

The PLS model (FIG. 7) explains the measured viscosities by 94% in 2comp\96% in 3 com. Cross validation reduces the explained variance toaround 60%.

From this it can be concluded that the recorded spectra at a givenflowrate relate to one point on the dynamic spectra curve. As done inthis experiment, running at four different flow rates (or using 4different dies) one can put the spectra together to characterize theflowcurve of the polymer.

EXAMPLE 2

Study Lot Variation Within a Single Product by Use of Acoustic Rheometer

It is well known that any commercial polymerisation process will besubject to certain variations in the molecular structure of the polymerproduced. The amount of variation is usually low and in some casesdifficult to quantify. Online methods are used to measure this variationin properties. The accuracy of the online rheometer will determine howwell small variations can be detected and thus in the long run avoided.To test the acoustic rheometer of the invention, 4 different lots of apolymer grades with known difference in molecular structure were testedusing the same setup as in example 1.

FIG. 8 shows the variation in viscosity of the 4 lots as measured bymeans of a rheometrics dynamic analyser in a frequency sweep mode (190°C. melt temperature).

Each sample was extruded at 4 rates (30,50,70.100 rpm on the 30 mmextruder). Spectra in the range 0-25 KHz were recorded on a Bruel &Kjaer acclerometer (nr 4384)

TABLE 2 Part-list (high temperature equipment) 1 Accelerometer 250° C.Brüel & Kjær, Number: 4384 Denmark 2 Coaxcables, 2 mm, Brüel & Kjær,Number: AO 0038 250° C. Denmark 1 Charge/Deltatron Brüel & Kjær, Number:2646 ampl. Denmark 1 UNF to BNC adapter Brüel & Kjær, Number: JP 0145Denmark 25 Cement studs Brüel & Kjær, Number: UA 0866 Denmark 25Extension Brüel & Kjær, Number: UA 0186 connectors Denmark

TABLE 3 Recording Parameters: Sampling frequency: 50 kHz Frequencyrange: 0-25 kHz Number of variables: 512 fewq. + 1 Temp. = 513 variableswindow size: 1024 data-points Transformation window Blackman Harristype: Number of replicates: 5 Recording length each 0.02 sec. replicate:Averages each replicate 100 spectrum unit: dBV rms

Data were pretreated as shown in FIG. 2 example 1. FIG. 9 shows a scoreplot of results: for each resin five replicates were run.

The data show the method to be able to separate between individual lotsfrom a commercial polymerisation.

1. A method of characterising a polymer under test, comprising: flowingsaid polymer through a conduit in a controlled manner such that saidpolymer experiences a predetermined flow rate, detecting acousticemissions from said polymer flowing in the conduit to provide acousticemission data relating solely to said polymer, and comparing theacoustic emissions data obtained against acoustic emission data from apolymer, or a series of polymers, of known characteristics, and-therebycharacterising the polymer.
 2. A method as claimed in claim 1 wherein arheological property of the polymer under test is thereby determined. 3.A method as claimed in claim 1 wherein the identity of the polymer undertest is thereby determined.
 4. A method as claimed in claim 2 whereinthe rheological property under test is the viscosity of the polymer. 5.A method as claimed in claim 1 wherein the acoustic emissions aredetected by means of an accelerometer.
 6. A method as claimed in claim 1wherein said conduit is associated with a pipe leading directly from apolymerisation reactor.
 7. A method as claimed in claim 1 wherein saidconduit is associated with an extruder.
 8. A method as claimed in claim1 wherein said conduit comprises a structural detail altering flowcharacteristics of the polymer flowing in the conduit.
 9. A method asclaimed in claim 1 wherein the acoustic emissions data is analysed usingPrincipal Component Analysis (PCA) techniques.
 10. An apparatus forcharacterising a molten polymer, comprising: a) means for controllingthe flow of said polymer through a conduit such that the polymerexperiences a predetermined flow rate; b) an acoustic sensor capable ofdetecting acoustic emissions from the polymer and thereby generating asignal relating solely to acoustic emissions data from said polymer; andc) means for comparing the signal against acoustic emissions data frommolten polymers of known characteristics.
 11. An apparatus as claimed inclaim 10, wherein said apparatus is adapted whereby to determine a valuefor a desired rheological property of the polymer under test.
 12. Anapparatus as claimed in claim 10, wherein said apparatus furthercomprises means for identifying the polymer.